Technical Abstract:
Studies were undertaken to investigate the cause of the bacteriostatic activity of fresh spinach leaves against Listeria monocytogenes. L. monocytogenes was cultivated in 1) pure tryptic soy broth used as control, 2) in tryptic soy broth containing 10 mg.ml-1 of autoclaved or non autoclaved freeze-dried spinach powder and 3) in tryptic soy broth in mixed dcultures with different microorganisms isolated from fresh-cut spinach which included Pseudomonas fluorescens biovar I (Ps. I), Ps. fluorescens biov. III (Ps. III), Staphylococcus xylosus (St.) and the total microorganisms. These microorganisms were inoculated at the concentration of 4.4 Log10 CFU.ml-1 and L. monocytogenes was inoculated at two concentrations, 2.4 or 4.4 Log10 CFU.ml-1. These results indicate that freeze-dried spinach powder had an inhibitory effect on the growth of L. monocytogenes. The inhibitory effect of spinach powder was more than 1.5 log of that of mixed cultures containing the low listerial inoculum. The inhibitory effect was greater when the initial number of L. monocytogenes were lower and were greater at the incubation temperature of 10C than 30C. The inhibitory effect decreased when the native microorganisms were almost eliminated by heating or irradiation. Among the native microorganisms, Ps. fluorescens biov. I had the greatest negative effect on the growth of L. monocytogenes. These results indicate that if fresh-cut spinach is contaminated with L. monocytogenes, its growth probably will be restricted by the natural microorganisms.